I need help with this rule:
When dealing damage, and applying it to armour values, do you take each point over as a point of damage, or is it each time you overwhelm the armour value?
Example:
A squad of 10 guys shoot at a tank using Mrk 1 guns, and do 30 damage. The tanks armor is a 15. Does the squad do 2 points of damage, or 15?

OverkillNormally, points of Damage in excess of a target's Armor rating are ignored. If an attack is so powerful that players think that even the excess Damage would be enough to overcome the target's Armor, then that excess amount can be treated as Overkill Damage. Especially powerful attacks may cause enough Overkill Damage to inflict multiple points of Size Damage, destroy several Components all at once, or even blow through multiple targets.

When an attacker makes a successful attack and decides to go for Overkill, he keeps track of the total Damage inflicted and the defender's unsuccessful Armor Roll against it. (If it looks like the attack might blow through multiple targets rather than simply dealing multiple layers of damage to a single target, he'll keep track of the initial Attack Roll as well - this is described further below.) After applying the appropriate General or Component Damage destruction from the initial attack, he then subtracts the result of the Armor Roll from the Damage done to find out how many points of Damage were "left over." This new total becomes Overkill Damage, and continues in the path of the original attack, doing more of the same type of damage (either General or Component Damage) to whatever objects it hits.

To answer your question, overkill would apply each time you overcome the armor value.

But in your example, only one point of damage would result. 15 is not enough to overwhelm the targets armor, 16 is. So that means you have 14 damage remaining, not enough for the second pip of damage.

When my friends and I play, apply new armor rolls for each wave of overkill, that way piss poor armor rolls (6 on 3d10) won't cause your awesome tank to blow up against one nice damage roll.

Pretty much every time your Damage Roll defeats the Armor, it damages one Structure Point. Subtract the previous Armor Roll/Value from the Damage Roll, and roll for Armor again. Every successful attack takes another Structure Point. Repeat until the Armor beats the remaining Damage.

D'oh! I for got that it is beating armour, not matching it. And for combined fire, onyx one damage roll is used, at least that's what I think. And what's the negative/ limitations on combined fire, besides the chance of the opponent rolling a high armour roll.